1. Field of the Invention
The present invention is broadly concerned with a method for preparing an asphalt composition useful for paving or road repair, and in particular, gelled asphalt-rubber compositions which provide an elastomeric matrix which has been found especially useful in paving applications while also reducing the solid waste problems attendant to disposal of used automotive tires. The invention also comprehends an improved, quick reaction time apparatus for the production of such gelled asphalt-rubber compositions which can if desired be of modular, mobile design permitting transport of the apparatus to a convenient location. In addition, the invention includes a method and apparatus for the preparation of foamed mixtures containing either asphalt and synthetic resin (e.g., polyethylene or polyethylene terephalate) or synthetic resin alone.
2. Description of the Prior Art
U.S. Pat. No. 3,891,585, describes elastomeric, gelled asphalt-rubber paving compositions which are made up of reacted fractions of paving grade asphalt and reclaimed rubber particles. These types of paving composition have proven to be very useful and successful, both in terms of wear characteristics and by virtue of the fact that manufacture thereof uses reclaimed rubber derived from used automotive tires. The latter factor is important inasmuch as such tires present a considerable solid waste problem, and their use in asphalt compositions provides both an economic and environmental advantage. Indeed, recent legislation has mandated that by 1997, at least 20% of federally funded highway asphalt paving projects will make use of asphalt-rubber.
As described in the '585 patent, paving grade asphalt is first heated to a temperature of from about 350.degree.-500.degree. F., followed by the addition of particulate rubber thereto. The resulting mixture is then thoroughly mixed in an atmospheric pressure mixer until the composition gels and is ready for mixing with aggregate. Generally speaking, the known process for obtaining asphalt-rubber compositions involves a reaction time on the order of 30-45 minutes with continuous agitation, until a dramatic viscosity change occurs, signalling that the gelation has developed. In order to obtain an essentially continuous output of gelled product, it is generally necessary to employ a series of large (4,500 gallon) agitated tanks, which represents a considerable cost. In addition, the gelled product is not storable, and therefore must be essentially produced on site. As a consequence, it is necessary to have the large mixing tanks on mobile platforms, which further increases costs.
It has also been known in the past to prepare foamed asphalts. Such materials are typically produced by injecting hot asphalt cement and cold water into an agitator containing cold wet aggregate. The asphalt remains in a foam state for a relatively short period of time (15-60 seconds) and is sprayed into the aggregate mixer. The resultant asphalt-aggregate mixture is then ready for storage or placement on a roadway. Foamed asphalts have also been produced by use of a rocket-type reactor feeding into an aggregate tank. In such a rocket-type reactor, respective streams of asphalt and water or steam are directed into a confined reaction zone where relatively high temperatures and pressures are generated, thereby creating a foamed product. The output from the rocket-type reactor is then applied to aggregate in the usual fashion. While foamed asphalts are advantageous, they do nothing to alleviate the solid waste problem of used automotive tires.
Modern industrial societies also generate tremendous quantities of scrap plastic or synthetic resin materials such as polyethylene (both high and low density) and polyethylene terephalate. It would be a material advancement in the art to provide a method and apparatus for reusing and reclaiming these plastics, either in the context of a foamed asphalt or as a foamed mixture per se.